ii8 THE BIOLOGY OF INSECTS 



" reduction divisions " during that maturation of the germ- 

 cells in both sexes which is a prelude to fertilisation. 



Sperm-cells are produced by the divisions of cells 

 known as spermatocytes which are the offspring of the 

 primitive germ-cells of the male insect. The primary 

 spermatocytes, like the primitive germ-cells, have the 

 number of chromosomes normal to the male of their kind 

 of insect. But when a primary spermatocyte is preparing 

 to divide into two secondary spermatocytes its chromosomes 

 become associated in couples (synapsis) and the members 

 of a couple, instead of splitting, separate from each other, 

 one passing into either nucleus of the two secondary 

 spermatocytes (Fig, 32, b). Thus each daughter- nucleus 

 receives only one chromosome of a pair and the chromosome 

 number is reduced to half that normal for the species. 

 Each secondary spermatocyte divides to form two sperma- 

 tids, the chromosomes splitting so that their reduced 

 number is maintained, this splitting being indeed in some 

 cases apparent before the completion of the first sperma- 

 tocyte division. Four spermatids are therefore formed as 

 the offspring of each primary spermatocyte, and up to this 

 stage in sperm-formation all the cells are minute globular 

 bodies. Then each spermatid becomes transformed into 

 an active sperm- cell (spermatozoon) with its compact 

 nuclear head and long vibratile tail or flagellum (Fig. 3, A) 

 ready to play its part in fertilising an egg-cell. 



The tgg has also a maturation process which it must 

 undergo in preparation for fertilisation. Immature eggs 

 (or primary oocytes) are the offspring of primitive germ- 

 cells not differing in aspect from the sperm-forming germ- 

 cells of the male. But the oocyte is much larger than the 

 spermatocyte because within it a quantity of yolk is stored 

 up to serve as food-supply for the growing embryo. As 

 regards the nucleus with its chromatin the maturation- 

 process of the tgg is essentially similar to that of the sperm. 

 The nucleus of the primary oocyte undergoes a reducing 

 division (meiosis), by which the nuclear material and the 

 number of chromosomes are reduced to half what they 



